Power inverters for converting DC power into AC power are typically of the polyphase type wherein a plurality of N AC phase outputs are produced. During operation of the inverter, an unbalanced load condition can arise wherein the loads on the phases are unequal. Typically, a polyphase filter is coupled to the inverter output which introduces phase shifts as a function of load. For such an inverter and filter, an unbalanced load condition can cause unequal phase shifts among the phases, resulting in phase errors.
The above described phase shifts are undesirable inasmuch as most polyphase loads require polyphase power at precise angular displacements.
The phase shifts are even more objectionable where the inverter is a part of a variable speed, constant frequency (VSCF) generating system which is connected in parallel with another VSCF system to one or more loads. In this case, the displacement between corresponding parallel connected phases of the VSCF systems determines the real load power share therebetween. If the displacements of the output phases of both systems are not controlled precisely, a highly undesirable load share condition may arise, in turn resulting in damage or destruction of one or more components.
Lafuze U.S. Pat. No. 4,218,732 discloses a phase angle balancing circuit for a VSCF system. Signals representing the polyphase output from a power converter are summed to produce a zero-sequence voltage signal which is combined in a negative feedback fashion with the control signals for switches in the power converter. Correction of phase shifts in the converter output is accomplished by a phase correction loop which vectorially sums correction voltages with each phase output wherein each correction voltage is at an angular displacement different than the phase output with which it is summed so as to reduce the angular displacement error. Such correction varies the magnitude of the converter output from a desired level, which variance is minimized by a voltage regulation loop. This balancing circuit, therefore, results in interaction between the phase correction and voltage regulation loops, in turn leading to increased complexity.